Dennis MacAlistair Ritchie’s was “dmac”, Bourne’s was “bourne”, Schmidt’s was “wendy!!!” (his wife’s name), Feldman’s was “axlotl”, and Kernighan’s was “/.,/.,”.

Source: Computer historians crack passwords of Unix’s early pioneers / Boing Boing

and Ken Thompson’s was “p/q2-q4!” (chess notation for a common opening move).